SMTP super slow - full SMTP queue

Hello all,

one of our customers is running an older Icewarp server 12.1.1. build 3 x64 running on Windows Server 2019 server. Everything went fine until today morning appro 8am.

There were no system updates or other changes on Icewarp or the system at all.

Til then if the Icewarp starts the SMTP queue runs full immediately up to more than 500 sessions.
The smtp log shows that almost every connection gets aborted with INCOMPLETE SESSION without any further information.
A test in Services > Server diagnostics brings 

Testing primary domain 'cse-online.de' for MX record...
Primary domain test successful.

Testing internet connectivity...
Internet connectivity test successful.

Testing internet services...
Internet services test failed.
The following services are not running: 
SMTP

Testing license server connectivity...
License server connectivity test successful.

Testing SMTP message transfer...
SMTP message transfer test failed.
Could not connect to mail server 'localhost'.

Because the servers session queue is full he doesn't accept any further connections and it is impossible to connect to the STMP service.

The STMP Service is running.

The servers overall performance is just great. It is only the Icewarp SMTP service not working as intended.

Sessions
Conspicuous is that there are 500+ServerSessions, and only 4-5 Client sessions.

Anyone has an idea why this happened and how we can solve this?

Thank you very much. Please let me know if you need further information.

Yours, Bernhard